1. Field of the Invention
This invention relates to a heavy load pneumatic radial tire which has plural of ribs divided by plural of main grooves in the circumferential direction on the tread surface of the tread portion and in more detail, relates to a technique of preventing uneven wear from being generated on the shoulder rib surface which runs on both sides of the tread surface of the said tread portion.
2. Description of the Prior Art
As heavy load pneumatic radial tires loaded on trucks, buses, or the like, which are driven on the well-paved road, as tread patterns, tires with ribs are employed more frequently than tires with blocks, from the viewpoint of wear resistance, ride quality, fuel cost, or the like. However, at the grounding part with loading, due to the bending of the side wall, particularly as for tires having ribs on the tread portion, due to the partially heavy pressure at the end part in the tire width direction of the shoulder rib, the abrasion of the part is promoted, and the shoulder rib wears earlier than the other ribs, thereby sometimes causing xe2x80x9cside dropped abrasionxe2x80x9d in which shoulder rib sinks like a step.
Such uneven wear begins at the external end part in the tire width direction in the shoulder rib of the tread portion and as the mileage increases, the uneven wear expands into the internal side in the tire width direction, and winds its way in the tire circumferential direction as well, thereby causing uneven wear which is so called xe2x80x9criver wearxe2x80x9d. When the mileage further increases, ribs in the shoulder portion as a whole are worn. Consequently, a step is generated between the adjacent ribs of the shoulder portion set inside in the tire width direction interposing the main groove therebetween, thereby causing xe2x80x9cside dropped abrasionxe2x80x9d. This xe2x80x9cside dropped abrasionxe2x80x9d is not preferable since it generates defective appearance and short duration of tire life.
Heretofore, in order to improve uneven wear, for example, a tire as shown in FIG. 7 has been provided. FIG. 7 shows an example of the conventional heavy load pneumatic radial tire and shows a schematic local sectional view of the right half in the tire width direction from the center line CL which vertically and equally divides the cross sectional view into two. FIG. 8 shows a schematic local sectional view of the tread portion showing the non-grounding state at the time of filling standard pressure in the conventional tire. FIG. 9 shows a schematic local sectional view of the tread portion showing the grounding state of the tire shown in FIG. 8. FIG. 10 shows an enlarged schematic local sectional view of the tread portion showing the grounding state near the shoulder portion shown in FIG. 9. FIG. 11 shows a schematic top view representing the tread pattern of the conventional tire.
A tire shown in FIG. 7 is provided with the carcass CC, the tread portion 100 which covers the crown region of the carcass CC, and the belt 106 having plural of belt cords between the tread 100 and the crown region of the said carcass CC. In addition, a tire shown in FIG. 7 is a tire having plural of main grooves 102 which run circumferentially at the tread surface 111 of the said tread portion 100 and plural of ribs 103 divided by these main grooves 102. In the tire in FIG. 7, as shown in FIGS. 7 and 8, a circular fine groove 150 is provided in the side region 140 A of the shoulder portion 140 running on both sides of the surface 111 of the tread portion 100 with the aperture 151 whose one end is opened to the side region 140 A. This fine groove 150 is the fine groove of straight groove sectional view having substantially the same groove width from the aperture 151 to the groove bottom 154. In other words, this fine groove 150 comprises the external wall 152 of the external side in the tire diameter direction leading to the external side 141 in the tire diameter direction of the side region 140 A of the shoulder portion 140, the internal wall 153 of the internal side in the tire diameter direction leading to the internal side 142 in the tire diameter direction of the side region 140 A of the shoulder portion 140, and the groove bottom 154 leading to the said external wall 152 and the said internal wall 153. The said external wall 152 and the said internal wall 153 extend to the groove bottom in a substantially parallel manner.
In this conventional tire, the said fine groove 150 decreases the partially heavy grounding pressure on the surface of the said rib 103 S, particularly on the surface of the end portion (rib 103 A) of the external side in the tire width direction since the force F to the external part in the tire width direction of the rib 103 S in the shoulder portion 140 which causes uneven wear, is diminished by the force required to fill the gap of the said fine groove.
However, when this conventional tire is filled with air at standard pressure and then loaded on the vehicle and driven, as shown in FIGS. 9 and 10, and when this tire contacts the ground surface G, the force F transmitted from the side wall SW which affects the rubber surrounding the said fine groove 150 makes the external wall 152 and the internal wall 153 of the fine groove 150 contact each other and acts as a force f1 indicated by arrow blocking in from the aperture 151 to the vicinity of the groove bottom.
Therefore, such force f1, as shown in FIG. 10, acts as the force to push the rib 103 S in the shoulder portion 140 in the grounding state into the internal side in the tire width direction (that is, the side of a center line which vertically and equally divides the cross sectional view of the tire into two), and in the surface of the said rib 103 S, its grounding pressure, particularly at the end of the tire width direction gets high on the ground surface. On the other hand, in accordance with the revolution of the tire at the time of driving, when the grounding state of the rib 103 S in the said shoulder portion 140 comes to be released, since the action by the force f1 against the said rib 103 S is gone, the fine groove 150 which has been compressed between the external wall 152 and internal wall 153 recovers to the initial state, the gains of the said grounding pressure on the surface of the said rib 103 S are released.
Thus, the above described phenomena are repeated in the conventional tire whenever the grounding and the non grounding state are repeated. Consequently, the conventional tire, as described above, although trying to prevent uneven wear, reducing the grounding pressure of the rib 103 S in the shoulder portion 140 by the said fine groove 150, right after the external wall 152 and the internal wall 153 in the fine groove 150 contact each other and block up, the grounding pressure begins to rise. Therefore, even if the conventional tire is provided with such fine grooves, it cannot be said that the tire fully prevents uneven wear which is likely to occur on the surface of the rib 103 S in the shoulder portion 140.
In addition, in the conventional tire, as described above, when it contacts the ground surface G, the force f1 makes the external wall 152 and internal wall 153 of the fine groove 150 contact and makes them block up from the aperture 151 to the vicinity of the back of the groove. Then by such force f1, the groove bottom 154 is compressed, leaving little gap of the fine groove 150, making the curvature of the groove bottom 154 big and making the deformation of groove bottom 154 big. And when the grounding state in this rib 103 S is released, the state recovers again to the state shown in FIG. 8 and the blocked-up state of the fine groove 150 is released. Therefore, since these phenomena are repeated at the time of driving, cracks are likely to be generated on groove walls, in particular, on groove bottom 154, and the tire life is likely to be damaged. These phenomena remarkably appear in the heavy load pneumatic radial tire used for trucks, buses, or the like, having heavy load on tires and driving on the well paved road for a long time and for a long distance.
The tire of the present invention is a heavy load pneumatic radial tire having a carcass, a tread portion which covers the crown region of the said carcass, a belt which comprises plural of belt cords between the tread portion and the said crown region of the said carcass, plural of main grooves running circumferentially on the tread surface of the said tread portion and plural of ribs divided by these main grooves, wherein:
the circular fine groove running in the tire circumferential direction and provided with an aperture at which one end is open to the side at one side of the said tire tread surface of the tread portion and at least at one side of the external region in the tire diameter direction of the side wall is provided;
the said fine groove has a narrowed area with the central axis of the sectional view of the groove sloping inward in the tire diameter direction from the said aperture to the internal part in the tire width direction and the groove width of the sectional view gets narrower as it goes from the said aperture to the back of the groove and an expanded area following thereafter whose groove width expands with a curvature;
the wall surface in the external diameter side of the narrowed area from the said aperture to the narrowest area of the said narrowed area slopes outward in the tire diameter direction than the central axis of the said sectional view of the groove and the wall surface in the internal diameter side of the said fine groove is provided with a convex part contacting the said wall surface in the external diameter side with pressure at the time of tire grounding;
when d represents the depth of the main groove nearest to the shoulder portion among the main grooves,
and B represents the largest width of the groove width of the said expanded area,
the bottom end of the groove at the innermost side in the tire diameter direction of the expanded area of the said fine groove is set nearer to the external side in the tire diameter direction than the position with the distance of (d+xc2xdB) from the tread surface of the tread portion to the internal side in the tire diameter direction; and
the bottom end of the groove at the innermost side in the tire width direction of the expanded area of the said fine groove is set nearer to the external side in the tire width direction than the position with the distance of d from the side of the said tire shoulder portion to the internal side in the tire width direction.
In the tire of the present invention, as described above, the said fine groove has a narrowed area with the central axis of the sectional view of the groove sloping inward in the tire diameter direction from the said aperture to the internal side in the tire width direction and the groove width of the sectional view gets narrower as it goes from the said aperture to the back of the groove and an expanded area following thereafter whose groove width expands with a curvature;
the wall surface in the external diameter side of the narrowed area from the said aperture to the narrowest area of the said narrowed area slopes outward in the tire diameter direction than the central axis of the said sectional view of the groove and the wall surface in the internal diameter side of the said fine groove is provided with a convex part contacting the said wall surface in the external diameter side with pressure at the time of tire grounding.
Therefore, when this tire is loaded on the vehicles and the rib in the shoulder portion contacts the ground, as a preferred embodiment, the said convex part contacts with pressure in the vicinity of the said narrowest area in the side wall surface of the external diameter sloping outward to the external side of the tire diameter direction than the central axis of the said sectional view of the groove, and the configuration of the expanded area prevents the contact between groove walls in the expanded area.
Since the tire of the present invention has the said configuration of the sectional view, the said convex part generates a slip after it contacts with pressure the said wall surface of the external diameter side of the fine groove and the convex part moves as if it goes into the relatively internal side in the tire width direction to the external side in the tire diameter direction than the fine groove. Therefore, the force F which is generated by the loading added to the tire is consumed as a friction energy and is transmitted as smaller force f1 and the convex part goes into the internal side in the tire width direction. By this, the deformation by bending of the side wall when the loading is added to the tire gets smooth and enlarges the diminution of the said force F similar to the case where the width of the fine groove in the conventional tire is enlarged. Thus, by taking this step, the grounding pressure of the shoulder rib in the external side in the tire width direction is further less likely to increase thereby capable of reducing the uneven wear of that part.
Moreover, in the tire of the present invention, the external wall and the internal wall of the fine groove contact each other in the vicinity of the said narrowest area at the time of driving. However, since the tire has the said groove sectional configuration, the groove as a whole does not block up to the groove bottom as seen in the conventional tire, and since the expanded area is formed with the groove width expanding with a curvature particularly at the groove bottom, in the configurational effect, too, the gap is not likely to get little as seen in the conventional tire and the curvature at the groove bottom is not likely to get large as seen in the conventional tire. Therefore, the crack initiation at the wall surface of the fine groove, especially at the groove bottom can be prevented, thereby enhancing the tire life.
Therefore, as a preferred embodiment, it is important that the expanded area of the fine groove of the present invention swells and expands in the internal side as well as the external side in the tire diameter direction and preferably expands with a curvature of a sectional arc or a substantial arc.
In addition, in the tire of the present invention, since the central axis of the groove sectional area of the fine groove slopes inward in the tire diameter direction from the said aperture to the internal side in the tire width direction and since the wall surface of the external side of the narrowed part from the said aperture to the narrowest part of the said narrowed part slopes outward in the tire diameter direction than the central axis of the said groove sectional area, even though the rib of the shoulder portion reduces to the internal side in the direction of tire diameter direction by a usual wear at the time of driving, the reduction in thickness at the end part of the shoulder portion in the tire diameter direction can be prevented. On the other hand, in the above described conventional tire, since the sectional view of the fine groove has a straight configuration, when the rib of the shoulder portion reduces to the internal side in the tire diameter direction by usual wear at the time of driving, the thickness in the sectional view of the rib in the tire diameter direction at the end part of the shoulder portion decreases, and the rib region in the external side in the tire external diameter direction of the fine groove, that is, the external side portion in the said tire diameter direction in the shoulder portion gets thin, thereby causing cracks or the like in the said part and it is not preferable from the viewpoint of durability. Since the tire of the present invention, compared with the conventional tire, can prevent the reduction in thickness in the external part in the said tire diameter direction, it enhances durability.
Although in the present invention, the position of the said fine grooves to be formed is not specifically limited, as a preferred embodiment, as described above, when d represents the depth of the main groove nearest to the shoulder portion among the said main grooves, and when B represents the largest width of the groove of the said expanded area, the bottom end of the groove at the innermost side in the tire diameter direction of the expanded area of the said fine groove is set nearer to the external side in the tire diameter direction than the position with the distance of (d+xc2xdB) from the said tread surface of the tread portion to the internal side in the tire diameter direction and the bottom end of the groove at the innermost side in the tire width direction of the expanded area of the said fine groove is set nearer to the external side in the tire width direction than the position with the distance of d from the said tire shoulder portion to the internal side in the tire width direction. The object of the present invention is to provide heavy load pneumatic radial tire capable of effectively preventing the crack initiation which is likely to occur at the groove wall or groove bottom provided in the side of the shoulder portion as well as further preventing the uneven wear appearing on the rib surface of the shoulder portion.